In many situations, it is desired to be able to provide an accurate and complete measurement of one or more periodically modulated signals using a receiver or measurement device whose bandwidth is less than the bandwidth of the periodically modulated signal.
For example, one may want to measure the error vector magnitude (EVM) of a power amplifier (PA), whereby the PA is excited by a contiguously aggregated 5-carrier LTE-A signal with a bandwidth of 100 MHz and a fundamental carrier frequency (fC) of 1.8 GHz. Because of spectral regrowth, the bandwidth of the fundamental PA output signal easily exceeds 300 MHz. Moreover, in the case of a broadband PA with higher-order intrinsic nonlinearities, the amplifier output signal will contain energy also around the harmonic carriers, which must also be measured.
FIG. 1 illustrates an example of such an amplifier output signal 110, including eleven harmonics Fc through 11FC. In general, as in the example of FIG. 11, the power level of higher order harmonics declines substantially. In many cases, only the first three or more harmonics have significant energy to be of concern. And in the case of the input signal to the amplifier, typically there is only significant energy in the fundamental frequency—the energy in the second, third, and other harmonics is typically −60 dBc or less with respect to the fundamental frequency.
Now we consider a case wherein the difference between the minimum and the maximum frequency contained in the spectrum of each harmonic exceeds the measurement bandwidth of the measurement instrument, and wherein the difference between the maximum frequency in one harmonic spectrum and the minimum frequency in the next harmonic spectrum also exceeds the measurement bandwidth of the measurement instrument.
FIG. 2 illustrates an example of this situation, in particular showing the first three harmonics 110-1, 110-2 and 110-3 of output signal 100 compared to an example measurement bandwidth RBW.
FIGS. 3A-3C illustrate in greater detail a problem with measuring the spectrum of such an output signal with a measurement instrument whose bandwidth is less than the bandwidth of any of the harmonics. For simplification of illustration and explanation, FIGS. 3A-3C illustrate measurement of only a single one of the harmonics of the output signal.
FIG. 3A shows an example frequency spectrum 320 of one harmonic of an output signal of a device under test (e.g., an amplifier) in response to a periodically modulated input signal.
FIG. 3B shows the example frequency spectrum 322 of the harmonic of the output signal of the device under test downconverted to baseband with the first mixing frequency LO1, together with the limited bandwidth RBW of a filter 330 of a receiver which is used to measure and characterize the periodically modulated input signal and the output signal of the device under test. Here it is assumed the bandwidth of the downconverted output signal of the device under test is SBW>RBW.
FIG. 3C shows the portion 324 of the spectrum of the harmonic of the output signal of the device under test which is actually able to be measured and characterized by the receiver with the limited bandwidth RBW. As denoted in FIG. 3B, a portion 323 of the harmonic of the spectrum of the output signal of the device under test is not measured by the receiver because of the limited bandwidth RBW.
Furthermore, since FIGS. 3A-3C only illustrate measurement of one harmonic of an output signal when it is desired to measure several harmonics of interest, the actual situation is much worse.
Thus it would be desirable to provide a convenient and reliable method and system to measure and characterize a periodically modulated signal, and an output signal of a device under test (DUT) produced in response to the periodically modulated signal, using a receiver whose bandwidth is less than the bandwidth of the periodically modulated signal itself and/or the bandwidth of the output signal. It would further be desirable to provide such a system and method which can provide accurate measurements of phase sensitive characteristics, such as the error-vector-magnitude (EVM), for a DUT. It would still further be desirable to provide such a system and method which can provide accurate measurements of phase sensitive characteristics, such as the error-vector-magnitude (EVM), for a DUT over multiple harmonics of the fundamental frequency.